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Map Projections Map Projection Map Projection Map Projections • Scientific method of transferring locations on Earth’s surface to a flat map • 3 major families of projection – Cylindrical • Mercator Projection – Conic Projections • Well suited for mid-latitudes – Planar Projections The Variables in Map Projection Map Projection Distorts Reality Projection Surface G P • A sphere is not a developable solid. • Transfer from 3D globe to 2D map mustresult in S Cyl loss of one or global characteristics: Light Source Varieties of – Shape geometric – Area O projections Cone – Distance – Direction – Position T O N Projection Orientation or Aspect We will come back to this graphic later in the lecture Characteristics of a Globe to consider Characteristics of globe to consider as as you evaluate projections you evaluate projections • Scale is everywhere the same: • Quadrilaterals equal in – all great circles are the same length longitudinal – the poles are points. extent formed a b between two • Meridians are spaced evenly along parallels have parallels. equal area. • Meridians and parallels cross at right angles. Area of a = area of b 1 Characteristics of globe to consider as you evaluate projections Classification of Projections: Pole • Areas of • What global characteristic preserved. quadrilaterals e formed by any d • Geometric approach to construction. two meridians c – projection surface and sets of b – “light” source evenly spaced parallels a • Orientation. decrease 0° poleward. 20° • Interface of projection surface to Earth. Area of a > b > c > d >e Global Characteristic Preserved Conformal Projections • Conformal • Retain correct angular relations in transfer from globe to map. • Angles correct for small areas. • Equivalent • Scale same in any direction around a point, but scale changes from point to point. • Equidistant • Parallels and meridians cross at right angles. • Large areas tend to look more like they do on the globe than is true for other projections. • Azimuthal or direction • Examples: Mercator and Lambert Conformal Conic Lambert Conformal Conic Projection Mercator Projection 2 Equivalent or Equal Area Equivalent or Equal Area Projections Projections • A map area of a given size, a circle three inches in diameter for instance, represents same amount of Earth space no matter where • A map area of a given size, a circle three on the globe the map area is located. inches in diameter for instance, represents same amount of Earth • Maintaining equal area requires: space no matter where on the globe the – Scale changes in one direction to be offset map area is located. by scale changes in the other direction. – Right angle crossing of meridians and parallels often lost, resulting in shape distortion. Maintaining Equal Area Mollweide Equivalent Projection Projection Pole Pole e d c b a 0° 0° 20° 20° Area of a > b > c > d >e Equivalent & Conformal Equidistant Projections • Length of a straight line between two OR Preserve true shapes and exaggerate areas points represents correct great circle Show true size and distance. squish/stretch shapes • Lines to measure distance can originate at only one or two points. 3 Plane Projection: Lambert Azimuthal Equal Area Azimuthal Projections North • Straight line drawn between two points depicts correct: – Great circle route – Azimuth • Azimuth = angle between starting point of a line and north q • Line can originate from only one point on map. q = Azimuth of green line Projection Azimuthal Centered on Rowan Projections Classified by Plane Projection Surface & Light Source Surface • Developable surface (transfer to 2D surface) • Earth grid and – Common surfaces: features • Plane projected from • Cone sphere to a • Cylinder plane surface. • Light sources: – Gnomonic – Stereographic – Orthographic 4 Plane Plane Projection: Lambert Azimuthal Equal Area Projection • Equidistant • Azimuthal Globe Projection to plane Conic Surface • Globe projected onto a cone, which is then flattened. • Cone usually fit over pole like a dunce cap. – Meridians are straight lines. – Angle between all meridians is identical. Equidistant Conic Projection Cylinder Surface • Globe projected onto a cylinder, which is then flattened. • Cylinder usually fit around equator. – Meridians are evenly spaced straight lines. – Spacing of parallels varies depending on specific projection. 5 Miller’s Cylindrical Projection “Light” Source Location • Gnomonic: light projected from center of globe to projection surface. • Stereographic: light projected from antipode of point of tangency. • Orthographic: light projected from infinity. Gnomonic Gnomic Projection Projection Gnomic Projection Stereographic Projection Mercator Projection 6 Stereographic Projection Stereographic Projection Orthographic Projection Projection Orientation • Orientation: the position of the point or line of tangency with respect to the globe. • Normal orientation or aspect: usual orientation for the developable surface: equator for cylinder, pole for plane, apex of cone over pole for cone [parallel]. • Transverse or polar aspect: – point of tangency at equator for plane. – line of tangency touches pole as it wraps around earth for cylinder. – Hardly done for cone • Oblique aspect: the point or line of tangency is anywhere but the pole or the equator. 7 Normal Orientation Mercator Projection Transverse Orientation Oblique Orientation Putting Things Together Projection Surface G P S Cyl Light Source Varieties of geometric O projections Cone T O N Projection Orientation or Aspect 8 Tangent & Secant Projections: Projection Surface to Globe Interface Cone • Any of the various possible projection combinations can have either a tangent or a secant interface: – Tangent: projection surface touches globe surface at one point or along one line. – Secant: projection surface intersects the globe thereby defining a: • Circle of contact in the case of a plane, • Two lines of contact and hence true scale in the case of a cone or cylinder. Tangent & Secant Projections: Projection Selection Guidelines Cylinder • Determine which global feature is most important to preserve [e.g., shape, area]. • Where is the place you are mapping: – Equatorial to tropics = consider cylindrical – Midlatitudes = consider conic – Polar regions = consider azimuthal • Consider use of secant case to provide two lines of zero distortion. Example Projections & Their Use • Cylindrical • Conic Cylindrical Projections • Azimuthal • Nongeometric or mathematical 9 Cylindrical Projections Cylindrical Projections • Equal area: • Cylinder wrapped around globe: – Cylindrical Equal – Scale factor = 1 at equator [normal aspect] Area – Meridians are evenly spaced. As one moves – Peters [wet laundry map]. poleward, equal longitudinal distance on the • Conformal: map represents less and less distance on the globe. – Mercator – Transverse – Parallel spacing varies depending on the Mercator projection. For instance different light sources • Compromise: result in different spacing. – Miller Peter’s Projection • Cylindrical • Equal area Central Perspective Cylindrical Mercator Projection • Light source at center of globe. • Cylindrical like mathematical projection: – Spacing of parallels increases rapidly toward – Spacing of parallels increases toward poles, but more poles. Spacing of meridians stays same. slowly than with central perspective projection. • Increase in north-south scale toward poles. – North-south scale increases at the same rate as the • Increase in east-west scale toward poles. east-west scale: scale is the same around any point. – Conformal: meridians and parallels cross at right – Dramatic area distortion toward poles. angles. • Straight lines represent lines of constant compass direction: loxodrome or rhumb lines. 10 Mercator Projection Gnomonic Projection • Geometric azimuthal projection with light source at center of globe. – Parallel spacing increases toward poles. – Light source makes depicting entire hemisphere impossible. • Important characteristic: straight lines on map represent great circles on the globe. • Used with Mercator for navigation : – Plot great circle route on Gnomonic. – Transfer line to Mercator to get plot of required compass directions. Gnomonic Projection with Great Circle Route Cylindrical Equal Area • Light source: orthographic. Mercator Projection with Great Circle Route • Parallel spacing decreases toward poles. Transferred • Decrease in N-S spacing of parallels is exactly offset by increase E-W scale of meridians. Result is equivalent projection. • Used for world maps. Miller’s Cylindrical • Compromise projectionà near conformal • Similar to Mercator, but less distortion of area toward poles. • Used for world maps. 11 Miller’s Cylindrical Projection Conic Projections Conic Projections Conics • Globe projected onto a cone, which is then • Equal area: opened and flattened. – Albers • Chief differences among conics result from: – Lambert – Choice of standard parallel. – Variation in spacing of parallels. • Transverse or oblique aspect is possible, but • Conformal: rare. – Lambert • All polar conics have straight meridians. • Angle between meridians is identical for a given standard parallel . Conic Projections Lambert Conformal Conic • Usually drawn • Parallels are arcs of concentric circles. secant. • Meridians are straight and converge on one • Area point. between standard • Parallel spacing is set so that N-S and E-W parallels is scale factors are equal around any point. “projected” • Parallels and meridians cross at right angles. inward to cone. • Usually done as secant interface. • Areas
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